The subject matter disclosed herein relates to variable speed control of a motor. More specifically, the subject matter disclosed herein relates to a method and apparatus for controlling the speed of a motor used in a material handling system as a function of the load applied to the motor.
Material handling systems are widely used to lift heavy loads, weighing up to hundreds of tons. A typical material handling system includes at least one motor used to raise and lower the load and at least one additional motor to position the material handling system over the load to be moved. Common applications include manufacturing facilities, in which large components may be positioned for assembly and/or the final assembly may be moved for shipping. In the shipping industry, containers are loaded and unloaded between storage facilities, trucks, trains, and ships. Such material handling systems typically include a hook block, connected by a cable to a drum, which is, in turn, connected directly or indirectly through a gearbox, to the motor. The hook may be directly connected to a lifting point on the object or indirectly connected to the object via another lifting apparatus such as chains, slings, or custom structures. The motor then rotates in one direction to raise the load and in the other direction to lower the load.
Still other material handling systems are designed to handle application specific requirements of certain loads. For example, bulk materials such as sand or salt may require a bucket, or clam shell, attachment. The clam-shell is lowered in an open position and closed to scoop up a load of the bulk material. Scrap yards and recycling facilities may utilize a magnet attachment which, when energized, may be used to pick up ferrous metals. Still other attachments are used as dictated by the application requirements. As described above, it is apparent that material handling systems are an integral part of many manufacturing processes. As a result, the efficiency of the material handling system impacts the efficiency of the manufacturing process.
It is known that electric motors are capable of producing rated torque up to rated speed and at least a portion of rated torque when operating above rated speed. The portion of rated torque the motor is capable of producing decreases the further above rated speed the motor operates. Therefore, in order to increase the efficiency of the material handling system, it is desirable to operate the motor at the maximum speed at which the motor can operate and still produce sufficient torque to control the load being moved.
Historically, it is known that the weight of the load may be determined by including a load sensor or by detecting the magnitude of the current in the motor. Such systems either weigh the load prior to operating the motor or detect a current once the motor is operating at constant speed. The maximum operating speed is then determined once during the initial operation of the motor and a maximum speed is set for the duration of the run. Such a system may be suitable for applications in which the load is constant; however, material handling systems may experience applications in which the load is changing. If the load decreases during operation, the motor no longer operates at peak efficiency, and if the load increases during operation, the motor may not be able to produce sufficient torque at the current speed to maintain control of the load.
Thus, it would be desirable to provide a system to improve productivity of material handling systems in which the load changes during operation.